1. Field of the Invention
The present invention relates to a method of stably performing a non-reactive sputtering method of forming a metal oxide film from a metal oxide target, over a long period of time. Particularly, the invention relates to a sputtering method of forming a metal oxide film on a metal film while controlling oxidation of the underlying metal film over a long period of time. More particularly, the invention relates to a sputtering method of varying amounts of active oxygen depending upon substrate positions (in other words, portions).
2. Related Background Art
Conventionally, there are well-known magnetron sputtering methods of forming the metal oxide film from the metal oxide target. Among them, proposals have been made to improve mechanical, electrical, or optical characteristics of the deposited film by controlling the amount of water vapor (H2O) present in an atmosphere during sputtering.
For example, Japanese Patent Application Laid-Open No. 61-64874 suggests, as a method of forming a metal oxide film, a technique of adding water vapor H2O to Ar gas, thereby preventing a stepped cut of a wiring pattern laid on the metal oxide film of an interlayer dielectric.
Japanese Patent Application Laid-Open No. 02-163363 suggests addition of water vapor H2O, or O2+H2O to Ar gas as a method of forming an ITO film. The Application describes that the addition of H2O, or O2+H2O is effective in improvement in electrical conductivity of ITO formed at substrate temperatures of room temperature to 200° C.
Further, Japanese Patent Publication No. 3007758 suggests, as a method of forming an ITO (Indium Tin Oxide) film, addition of one of N2, SiH4, NH3, and PH3 to Ar gas, in order to overcome the problem of unstable transparency and film thickness of the ITO film due to adsorption of O2 to the target in the previous O2 addition methods. It is described that the method was successful in achieving stable transparency and film thickness.
Meanwhile, it is disclosed that when H2O or H2 is added in order to stabilize etching rates of the ITO film formed over a long period of time, the etching rates can be maintained constant, independent of film forming durations (0-3 hours). (Plastics Processing Technologies (Plastics Kako Gijutsu), p12-p18 and FIG. 5, Vol. 17, No. 3 (1990))
Concerning formation of a back reflecting layer of a solar cell, U.S. Pat. No. 4,419,533 discloses a technique of forming a barrier layer of zinc oxide or the like so as to prevent the reflecting layer from diffusing into a photoelectric conversion layer. Specifically, a transparent conductive layer of a metal oxide or the like is interposed between the reflecting layer and the semiconductor layer, thereby preventing the diffusion of zinc oxide.
Japanese Patent Application Laid-Open No. 10-310862 (correspondent of U.S. Pat. No. 5,998,730), which was also filed by the Assignee of the present application, describes a technique of heating and cooling a film forming chamber, thereafter forming a metal reflecting film on a substrate, bringing the film into contact with active oxygen, and then forming a transparent conductive film thereon, by which the reflecting film and the transparent conductive film can be stably formed in desired texture structure, with high reflectance, at low cost, and with high reliability. Namely, the interface is positively oxidized between the metal layer and the metal oxide layer, thereby preventing “oxidation of the metal layer by the metal oxide layer” and “melanization due to reduction of the metal oxide layer.” The control on the amount of active oxygen is employed for oxidation of a metal target by the reactive sputtering method, but is normally not employed for the production from the metal oxide target by the non-reactive sputtering method.
The foregoing conventional example (Plastics Processing Technologies) discloses the technique of maintaining the characteristics of the ITO film constant throughout the film forming duration of three hours, but fails to disclose any technique of forming a stable metal oxide film for maintaining characteristics of the semiconductor element such as the solar cell or the like constant throughout long-term film formation.
It is generally known that the addition of water vapor in the above conventional example involves the following problems. Namely, the problems are:    1. need for a water vapor generator by bubbling,    2. need for temperature management so as to avoid dew condensation in a pipe from the water vapor generator to the film forming chamber,    3. need for a device for monitoring the condition of water vapor inside the film forming chamber,    4. need for means for avoiding influence of water vapor adhering to the monitoring device itself, and so on.
There is also a method of monitoring the interior of the discharge space with a quadrupole mass spectrometer or the like and controlling the amount of water vapor introduced into the vacuum chamber, but it lacks precision. A conceivable reason for it is that it is affected by adsorption and dew condensation of water on the water adding means and the monitoring device.
Further, even in film formation under constant discharge current, voltage, and power in a back reflecting layer producing system (hereinafter also referred to as BR apparatus) detailed hereinafter, long-term sputtering film formation of metal/metal oxide films on a beltlike substrate (note: a beltlike substrate wound in a roll form will also be referred to as a roll substrate) can result in forming a back reflecting layer (hereinafter also referred to as a BR layer) with different surface textures at an initial position and at a last position of the beltlike substrate, and formation of a solar cell thereon does not readily allow the solar cell to be formed with Jsc (short-circuit current) constant.
It is mentioned in general that the optical confinement function of the surface texture lengthens optical path lengths of incident light to facilitate absorption of the light in the photovoltaic layer of the solar cell. Accordingly, the surface texture departing from the desired shape must result in reduction of Jsc.